The present invention relates generally to the deposition of pyrolytic carbon in fluidized bed converters and more particularly to processes and apparatus for coating substrates in a fluidized bed so as to achieve highly uniform crystalline pyrocarbon coatings thereupon.
The process of forming pyrolytic carbon, i.e. pyrocarbon, coatings upon substrates utilizes high-temperature gas-phase dissociation of low molecular weight, saturated or unsaturated, hydrocarbons which undergo polymerization and/or dehydrogenation reactions so as to ultimately form a finely crystalline form of carbon. It is well known to coat substrates with pyrocarbon for various different purposes, and when it is desired that such pyrocarbon coatings completely envelop the substrate in question, the use of a fluidized bed levitation process has become the process of choice.
Investigations have been carried out for over 30 years with respect to the production of pyrocarbons of different characters in fluidized beds, and during this period of time, it has been found that the crystalline character of the pyrocarbon is dependent upon a large number of parameters including deposition temperature, hydrocarbon composition and percentage, contact time of the gas stream with the substrates, and the ratio of surface area to volume in the chamber where coating is taking place. These investigations have determined that the crystalline character and the mechanical strength of the resultant pyrocarbon can be altered by careful manipulation of the foregoing parameters in a fluidized bed process, and there are many U.S. patents which discuss such parameters, e.g. U.S. Pat. Nos. 3,325,363 and 3,547,676.
As a part of these investigations and studies, it was discovered that pyrocarbon of a specific character, i.e. having a relatively high density of at least about 1.5 gm/cm3, having an apparent crystallite size of about 200 xc3x85 or less and having high isotropy, exhibited remarkedly good properties for use in devices that would have direct or indirect contact with blood in the circulatory system of humans because such pyrocarbon was outstandingly inert and did not give rise to thrombosis. Such pyrocarbon coatings generally became the materials of choice for prosthetic heart valves and other components which would be exposed to the human blood, and U.S. Pat. No. 3,685,059 is indicative of such pyrocarbons. During these early periods, it was felt that such pyrocarbons, in order to provide the desired hardness and high structural strength, should be alloyed with a carbide-forming element, such as the metalloid silicon which would form silicon carbide as a part of the pyrocarbon deposition process. More recently, it has been found that, by depositing pyrocarbon in a fluidized bed under very carefully controlled conditions, as taught in U.S. Pat. No. 5,514,410, pure unalloyed pyrocarbon coatings having high fracture toughness, high strength and high strain-to-failure, as well as adequate hardness, can be created.
Relatively early in the study of such processes for the fluidized bed deposition of pyrolytic carbon, it was determined that the changing character of the usual bed over the length of a normal pyrocarbon coating run, as a natural result of the growing diameters of the small particles in the bed, could have a significant effect upon the crystalline structure of the pyrocarbon being deposited. In this respect, 1976 U.S. Pat. No. 3,977,896 taught an improved process for pyrocarbon deposition in a fluidized bed wherein, through the selective removal and addition of the fluidized particles, the total deposition area within the coating enclosure could be maintained relatively constant, and such would provide improved uniformity in the crystalline characteristics of the pyrocarbon being deposited.
Such careful bed control has continued to be investigated since that time, see for example U.S. Pat. No. 4,546,012. Improvements were made such as are disclosed in U.S. Pat. Nos. 5,284,676 and 5,328,713 where constant monitoring of conditions within the fluidized particle bed is carried out and used to substantially continuously adjust the bed to maintain the precise coating bed character. This results in a precise rate of coating, allowing the thickness of the pyrocarbon deposit to be extremely carefully controlled.
These investigations in processes for the fluidized bed coating of substrates with pyrocarbon have likewise varied the manner of supply of the coating atmosphere to the chamber in which pyrocarbon deposition is occurring. In this respect, the atmosphere is usually a mixture of a vaporous carbonaceous substance and an inert gas which is supplied at a sufficient rate to provide the desired fluidizing effect. Generally, a relatively low molecular weight hydrocarbon, such as propane, propylene, methane or the like, is supplied along with an inert gas, such as nitrogen, argon, helium or the like, through a gas injection arrangement located at the bottom of a vertically oriented chamber wherein the pyrocarbon deposition occurs, as shown in the ""896 patent, the ""410 patent, the ""676 patent, and the ""713 patent. The gas injection method of recent choice utilizes a lower nozzle arrangement wherein a gas supply passageway is located on the axis of a generally right circular cylindrical coating chamber and leads to a conical nozzle region which opens smoothly to the full diameter of the cylindrical deposition chamber. Such a construction has generally been the nozzle construction of choice during about the last two decades for depositing pyrocarbon coatings upon small components, such as valve bodies and occluders for heart valves, although some other arrangements have been disclosed and presumably utilized to some extent. The operation of such construction of choice is generally illustrated in the ""676 patent, where it is shown that the ascending portion of the fluidized bed is located vertically above the incoming gas passageway, with the returning particles in the bed descending along the outer periphery and then being directed, by the conical shape of the nozzle region, back into the center where they are again levitated upward. This arrangement is commonly referred to as a spouting bed.
As examples of other proposed gas injection arrangements, reference is made to the following U.S. patents. U.S. Pat. No. 5,328,720 discloses the use of a substantially flat grid, which contains a plurality of essentially uniformly disposed straight passageways, located above a distribution-type entrance region. U.S. Pat. No. 3,889,631 shows a variety of gas distribution plates with passageways of various exit shapes that are distributed symmetrically about the area of the plate, and U.S. Pat. Nos. 3,398,718 and 4,387,120 are generally similar. U.S. Pat. No. 3,636,923 discloses the more common conical nozzle region; however, it also incorporates a more elaborate gas distributor, as best seen in FIG. 2. U.S. Pat. No. 4,342,284 shows a variety of gas injectors which have some general similarity to the arrangement shown in the ""923 patent. U.S. Pat. No. 4,335,676 discloses another type of injector for introducing a gaseous flow into the bottom of a spouting bed coating chamber of this general type wherein the particles move upward in the form of a spout or geyser and then fan radially out toward the surrounding wall of the enclosure; it includes the usual conical section to accommodate the return of the descending particles in the bed. U.S. Pat. Nos. 3,566,830 and 4,221,182 disclose other arrangements which bear a general similarity to that shown in the ""676 patent.
U.S. Pat. No. 4,080,927 discloses an alternative arrangement to that shown in the ""182 patent which is adapted for coating small spheroids with pyrocarbon in a fluidized bed. A levitating gas, such as argon, helium or nitrogen, is injected separately into the coating chamber through a plurality of passageways 14 (supplied via an entrance chamber 28), while the reactant gas, i.e. a hydrocarbon, is injected through a central nozzle 18 having a plurality of discharge passageways 20. A variety of different nozzles are disclosed in FIGS. 6-11. U.S. Pat. No. 3,940,514 shows a fluidized bed coater where a bed of particles having the usual spouted bed configuration are coated with a liquid which is being injected into and atomized within a coating chamber.
Although various of the aforementioned improvements have rendered the process of coating with pyrocarbon far more efficient, the search has continued for still better processes, particularly ones which facilitate the deposition of highly uniform crystalline pyrocarbon.
Processes and apparatus for depositing pyrolytic carbon in a fluidized bed in a generally vertically oriented coating chamber are provided which are efficient and which result in the deposition of highly uniform crystalline pyrocarbon upon the substrates. The usual gaseous atmosphere, containing an inert carrier gas and a hydrocarbon or the like, is injected into the chamber at a lower location, and the chamber is heated to a temperature such that the hydrocarbon pyrolytically decomposes at the surfaces of the substrates being levitated in the fluidized bed, depositing pyrocarbon thereupon. The injection of the gaseous atmosphere is carried out in a manner so that an ascending fluidized bed region is established along one boundary portion of the chamber and a descending fluidized bed region is established along an opposite boundary portion of the chamber; this flow pattern is referred to as asymmetric and departs radically from the typical symmetrical spouting bed arrangement that has been employed for decades. As a result, there is minimization of stagnant locations within the fluidized bed portion of the coating chamber and a faster transit of the substrates being coated through the regions of the bed where temperature and gas concentration gradients inevitably occur, as a result of which the pyrocarbon coatings are deposited which are essentially free of discontinuities such as minor voids, cracks and soot bands and have highly uniform crystallinity.